The invention described herein may be manufactured, used and licensed by or for the U.S. Government.
1. Field of the Invention
The invention is directed to an apparatus and method for detecting low concentrations of a smoke having a small particle size (e.g., 10 microns).
2. Description of Related Art
There is a need in the art to detect smoke having low concentrations and a small particle size (e.g., 10 xcexcparticles of carbon or brass). Conventional detection of such smoke has relied on optical detection using visible, infrared, or ultraviolet light. However, such optical detection is dependent on the particles"" ability to scatter, absorb, or forward scatter light and have limited performance in extremely low concentrations of smoke.
Co-pending application Ser. No. 09/400,146, filed Sep. 12,1999, entitled xe2x80x9cDevice and Method for Inspection and Detection of a Material by Observing a High-Voltage Waveform Produced by that materialxe2x80x9d, teaches detecting low impedance fibers by providing a high voltage between two brass screens and observing an arc when a fiber bridges the gap between the screens. However, this technique is not well suited to detecting small particles, as the gap between the screens is far too large. Flat-plate designs used in corona-discharge systems for spectroscopy are similarly unsatisfactory.
It is an object of the invention to detect the presence of smoke (e.g., particles of carbon or brass) in a fluid, particularly in air.
It is another object of the invention to detect the presence of smoke in a large volume of air.
It is a further object of the invention to detect the presence of smoke with a reduced lag time while detecting the smoke in an enclosure.
It is a further object of the invention to detect the presence of smoke in a device with a small pressure drop so as to conserve power and reduce noise.
To achieve these and other objects, the present invention is directed to a detector for detecting particles in a fluid, the detector comprising: a voltage supply for supplying an adjustable voltage; a first electrode having at least one interior opening extending through the first electrode; and a second electrode comprising at least one pin extending through the at least one interior opening to define at least one passageway for the fluid; the first electrode and the second electrode being connected to the voltage supply so that the adjustable voltage is applied between the first electrode and the second electrode. The present invention is further directed to a detector for detecting particles in a fluid, the detector comprising: a voltage supply for supplying a voltage; a first electrode having an interior opening extending through the first electrode; and a second electrode comprising a pin extending through the interior opening to define a passageway for the fluid; the first electrode and the second electrode being connected to the voltage supply so that the voltage is applied between the first electrode and the second electrode to define an electric field between the first electrode and the second electrode; and at least one of the interior opening and the pin being shaped so that the passageway is tapered and the electric field varies along the length a method of detecting particles in a fluid, the method comprising: (a) providing a detector comprising (i) a voltage supply for supplying a voltage, (ii) a first electrode having at least one interior opening extending through the first electrode, and (iii) a second electrode comprising at least one pin extending through the at least one interior opening to define at least one passageway for the fluid, the first electrode and the second electrode being connected to the voltage supply so that the voltage is applied between the first electrode and the second electrode to produce an electric field between the first electrode and the second electrode; (b) introducing the fluid into the passageway; (c) applying the voltage from the voltage supply to the first electrode and the second electrode; (d) varying the electric field; (e) determining a value of the electric field at which an arc occurs in the fluid; and (f) detecting the particles in accordance with the value of the electric field determined in step (e).
The invention works on the principle that the discharge threshold, which is the potential difference required for a breakdown of an electrical field (E field) to produce an arc, is dependent on the conductivity of the sum of the impedances of the media between the opposing electrodes used to apply the potential difference and on the distance between the electrodes. The conductivity is influenced by the base medium (such as air), the presence of foreign particles in the base medium (such as carbon particles in smoke), and the size and composition of the particles. If the base medium is uniform (as is the case with air and water), it is the particles which cause the change in the discharge threshold. Since the effects of the particles are determined both by their size and by their conductivity, particles in a stream can be analyzed. If the material is contaminated, e.g., with water, the conductivity changes, and so does the threshold.
A design featuring a cylinder with a rod or pin in the center, both of which function as electrodes, and a design featuring a honeycomb arrangement of such cylinders and rods or pins allow a large cross-sectional area for air flow and thus are relatively unobtrusive to the fluid flow while minimizing the distance between the electrodes. Thus, sensitivity is improved, while high flow rates and low pressure drop are maintained.
The discharge threshold also depends on the particle size. If it is important to detect particle size as well, the E field can be set so as to detect particles above or below a cutoff size by determining whether the arc occurs. The E field can be varied during such detection, and the value at which the arc starts or stops allows a determination of the particle size.
The limit setting of the E field is dependent on the particle size and conductivity. In a stream of unknown particles, this dependence allows a level of filtering particle types by varying the E field.
A preferred embodiment of the invention uses highly charged conductive probes (electrodes) spaced apart to form an E field and senses the voltage required to cause an arc. The voltage between the electrodes, and with it the E field, can be increased or decreased, and knowledge of the voltage at which arcing occurs allows a determination of the content (chemical composition of the particles) if the particle sizes and concentrations are known. In the case of particles having a known chemical composition, the size or concentration of the particles can be determined. These determinations are possible because particles of the same size and different compositions arc at different voltages, as do particles of different sizes and the same composition. An experiment was conducted with cigarette smoke, in which the difference between inhaled and uninhaled smoke could be seen.
The device and method according to the present invention are highly sensitive to all kinds of conductive particles and is especially valuable for small particles. The present invention is not dependent on the particles"" ability to scatter, absorb, or forward scatter light in the IR, visible, or UV range.
Conductive particles have uses in both governmental and commercial areas. Besides detection and identification of obscurants in military applications, the invention can be used to detect contaminants in water or another fluid.
While it is contemplated that the invention will be used with an air flow, any fluid can be used.